Session initiation protocol (SIP) protocol is employed to initiate and to modify a multimedia session over the internet protocol (IP) network. For example, SIP is used in one of a universal mobile telecommunications system (UMTS) network to establish a multimedia session between two users wherein at least one of the users operates on the UMTS system. However, SIP protocol has not heretofore been utilized as a mechanism to perform handoff between different devices operating in different systems.